Problems with Parents/If you're not the parent
I have a half-brother who is 5 and a half-sister who is 4 and their dad has a very short temper. He yells at them for small mistakes they make and only stops when they're crying, hiding their faces, and completely ashamed. He gets mad when I comfort them saying they need to learn. He doesn't really hit them, but will sometimes grab their arm and squeeze it or smack them on the leg. I've tried telling my mom to talk to him and it seems like anything she says does not go through to him. And she doesn't bother to try any further unless I bring it up, then she gets very defensive saying he could be worse. Also, she threatens my sister and brother saying she'll tell their dad they've misbehaved when he's not around. I really feel I can't talk to their dad about the situation. He too gets very defensive and last time I tried to talk to him it didn't help. I want to help my brother and sister because I really care about them. Sometimes they get really down. My sister's told me she's afraid of her dad sometimes. I don't know what to do. I'm not the parent, but I hate to stand by and do nothing. My brother is really funny and artistic when his father's not around, but when he's there he becomes more careful and sad. But they still love their dad and they do have good days. It's not always bad. It's just so confusing when some days are so bad and it's all I can think about.
I understand how you feel. men have little patience with young kids to begin with and your mom probably feels that she should only have to tell him once to take it easy.
Chances are that he feels that since you are one of the children in the family that you don't understand, so he looses patience once again and that only makes matters worse.
Do as you are doing and when happens, just be there for the kids so they have someone to talk to and go to with their problems. Your step dad will get over this as the kids get older. Try to keep them outside or playing something so they don't get on his nerves.
It is hard to answer this without knowing how your daily family routine is. It may pay to talk to your minister or and aunt or grandma and get their opinion. But, only if they are sworn to secrecy so you don't get into trouble.
I wish I could be of more help, but this will take someone closer to the situation to help to guide you through. I hope these tips helped you a little.